Can't undef active subroutine

[philiprbrenan]

Should this produce: "deep recursion" not: "Can't undef active subroutine" ?

#!/usr/bin/perl
sub a
 {my sub b
   {&a; # Can't undef active subroutine
   }
  b
 }
a

[tonycoz]

I've reproduced this and I see what's happening.

Lexical subs have the SV* for their pad entry copied into the new pad (with an SvREFCNT_inc()), rather than a new SV being created, like other lexicals.

The leads to a more subtle problem too:

$ ./perl -le 'sub foo { my $x = shift; my sub b { $x }; foo(2) if $x == 1; return b() } print foo(1)'
2

Note that the outer b() (bound to where $x is 1) is called, but the inner recursive b() instance (where $x is 2) is used instead, for example with this non-lexical-sub equivalent:

$ ./perl -le 'sub foo { my $x = shift; my $b = sub { $x }; foo(2) if $x == 1; return $b->() } print foo(1)'
1

[jimav]

Hi, Any thoughts about what to do about this?

If I understand, lexical subs can not currently be used with recursion which re-enters the scope where the lexical sub is defined, because the recursive call will not create a new closure (or not properly). Is that right?

If there is no solution possible, then it might be best to document the limitation in perlsub.

#!/usr/bin/env perl
use strict; use warnings; use feature qw/say state/;
STDOUT->autoflush; STDERR->autoflush;
say "Perl version $^V";

sub process($;$) {
  my ($value, $callback) = @_;
  say "### process($value,", ($callback//"undef"), ")";

  my sub inner {
    if ($callback) {
      return $callback->($value+1);
    } else {
      return $value+1;
    }
  }

  inner();
}

my $result =
  process(1, sub{
                 my $arg = shift;
                 say "#   callback called with arg $arg";
                 process($arg+100);
              });
say "result=", ($result//"undef");

#  Perl version v5.36.0
#  ### process(1,CODE(0x555555971f10))
#  #   callback called with arg 2
#  Can't undef active subroutine at /tmp/t line 25.

[tonycoz]

I'll take another look, I suspect now I was trying to solve some non-existent problems last time I tried to fix this.

[jkeenan]

I'll take another look, I suspect now I was trying to solve some non-existent problems last time I tried to fix this.

@tonycoz, have you been able to look into this problem any further?